Fast envelope tracking systems for power amplifiers

ABSTRACT

Fast envelope tracking systems are provided herein. In certain embodiments, an envelope tracking system for a power amplifier includes a switching regulator and a differential error amplifier configured to operate in combination with one another to generate a power amplifier supply voltage for the power amplifier based on an envelope of a radio frequency (RF) signal amplified by the power amplifier. The envelope tracking system further includes a differential envelope amplifier configured to amplify a differential envelope signal to generate a single-ended envelope signal that changes in relation to the envelope of the RF signal. Additionally, the differential error amplifier generates an output current operable to adjust a voltage level of the power amplifier supply voltage based on comparing the single-ended envelope signal to a reference signal.

CROSS-REFERENCE TO RELATED APPLICATION

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet, or any correction thereto,are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND Field

Embodiments of the invention relate to electronic systems, and inparticular, to power amplifiers for radio frequency (RF) electronics.

Description of the Related Technology

Power amplifiers are used in RF communication systems to amplify RFsignals for transmission via antennas. It is important to manage thepower of RF signal transmissions to prolong battery life and/or providea suitable transmit power level.

Examples of RF communication systems with one or more power amplifiersinclude, but are not limited to, mobile phones, tablets, base stations,network access points, customer-premises equipment (CPE), laptops, andwearable electronics. For example, in wireless devices that communicateusing a cellular standard, a wireless local area network (WLAN)standard, and/or any other suitable communication standard, a poweramplifier can be used for RF signal amplification. An RF signal can havea frequency in the range of about 30 kHz to 300 GHz, such as in therange of about 450 MHz to about 6 GHz for certain communicationsstandards.

SUMMARY

In certain embodiments, the present disclosure relates to mobile device.The mobile device includes a transceiver configured to generate a radiofrequency transmit signal, a front end circuit including a poweramplifier configured to amplify the radio frequency transmit signal, anda power management circuit including an envelope tracker configured togenerate a power amplifier supply voltage of the power amplifier. Theenvelope tracker includes a differential envelope amplifier configuredto amplify a differential envelope signal to generate a single-endedenvelope signal that changes in relation to an envelope of the radiofrequency transmit signal, and a differential error amplifier configuredto generate an output current based on comparing the single-endedenvelope signal to a reference signal. The output current is operable toadjust a voltage level of the power amplifier supply voltage.

In some embodiments, the differential envelope amplifier includes anamplification circuit configured to amplify the differential envelopesignal, and a common mode feedback circuit operable to compensate theamplification circuit for a common mode error arising from a common modevoltage of the differential envelope signal. According to a number ofembodiments, the amplification circuit includes a first differentialinput configured to receive the differential envelope signal and asecond differential input configured to receive a differentialcompensation signal from the common mode feedback circuit. In accordancewith several embodiments, the common mode feedback circuit is configuredto provide feedback from an output of the amplification circuit to thesecond differential input of the amplification circuit. According tovarious embodiments, the envelope tracker further includes adifferential input filter configured to filter the differential envelopesignal prior to amplification by the amplification circuit.

In a number of embodiments, the differential error amplifier includes afirst input configured to receive a reference voltage and a second inputconfigured to receive the single-ended envelope signal, and the secondinput has an input impedance this is lower than an input impedance ofthe first input. In accordance with various embodiments, the inputimpedance of the second input is at least ten times lower than the inputimpedance of the first input.

In several embodiments, the envelope tracker further includes aswitching regulator configured to generate a regulated voltage, and acombiner configured to combine the regulated voltage and the outputcurrent to thereby generate the power amplifier supply voltage. Inaccordance with a number of embodiments, the differential erroramplifier is further configured to generate a sense signal that tracksthe output current, the switching regulator configured to generate theregulated voltage based on the sense signal.

In certain embodiments, the present disclosure relates to an envelopetracking system. The envelope tracking system includes a power amplifierconfigured to amplify a radio frequency signal, and an envelope trackerconfigured to generate a power amplifier supply voltage of the poweramplifier. The envelope tracker includes a differential envelopeamplifier configured to amplify a differential envelope signal togenerate a single-ended envelope signal that changes in relation to anenvelope of the radio frequency signal, and a differential erroramplifier configured to generate an output current based on comparingthe single-ended envelope signal to a reference signal. The outputcurrent is operable to adjust a voltage level of the power amplifiersupply voltage.

In some embodiments, the differential envelope amplifier includes anamplification circuit configured to amplify the differential envelopesignal, and a common mode feedback circuit operable to compensate theamplification circuit for a common mode error arising from a common modevoltage of the differential envelope signal. According to a number ofembodiments, the envelope tracker further includes a differential inputfilter configured to filter the differential envelope signal prior toamplification by the amplification circuit.

In various embodiments, the differential error amplifier includes afirst input configured to receive a reference voltage and a second inputconfigured to receive the single-ended envelope signal, and the secondinput has an input impedance that is lower than an input impedance ofthe first input.

In several embodiments, the envelope tracker further includes aswitching regulator configured to generate a regulated voltage, and acombiner configured to combine the regulated voltage and the outputcurrent to thereby generate the power amplifier supply voltage. Inaccordance with some embodiments, the differential error amplifier isfurther configured to generate a sense signal that tracks the outputcurrent, and the switching regulator is configured to generate theregulated voltage based on the sense signal.

In certain embodiments, the present disclosure relates to a method ofenvelope tracking. The method includes amplifying a radio frequencysignal having an envelope using a power amplifier, and generating apower amplifier supply voltage of the power amplifier using an envelopetracker. Generating the power amplifier supply voltage includesamplifying a differential envelope signal to generate a single-endedenvelope signal that changes in relation to the envelope using adifferential envelope amplifier, generating an output current bycomparing the single-ended envelope signal to a reference signal using adifferential error amplifier, and adjusting a voltage level of the poweramplifier supply voltage using the output current.

In a number of embodiments, the method further includes amplifying thedifferential envelope signal using an amplification circuit of thedifferential envelope amplifier, and compensating the amplificationcircuit for a common mode error arising from a common mode voltage ofthe differential envelope signal using a common mode feedback circuit.According to various embodiments, the method further includescompensating the amplification circuit for the common mode errorincludes receiving the differential envelope signal at a firstdifferential input of the amplification circuit, outputting thesingle-ended envelope signal at an output of the amplification circuit,and providing feedback from the output of the amplification circuit to asecond differential input of the amplification circuit. In accordancewith several embodiments, the method further includes filtering thedifferential envelope signal prior to amplifying the differentialenvelope signal using the amplification circuit.

In some embodiments, generating the power amplifier supply voltagefurther includes generating a regulated voltage using a switchingregulator, and combining the regulated voltage and the output current tothereby generate the power amplifier supply voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a communicationsystem for transmitting radio frequency (RF) signals.

FIG. 2A is a schematic diagram of one embodiment of an envelope trackingsystem for a power amplifier.

FIG. 2B is a schematic diagram of another embodiment of an envelopetracking system for a power amplifier.

FIG. 3 is a schematic diagram of another embodiment of an envelopetracking system for a power amplifier.

FIG. 4 is a schematic diagram of one embodiment of a differentialenvelope amplifier for an envelope tracking system.

FIG. 5 is a schematic diagram of another embodiment of a differentialenvelope amplifier for an envelope tracking system.

FIG. 6 is a schematic diagram of one embodiment of an amplificationcircuit for the differential envelope amplifiers of FIGS. 4 and 5 .

FIG. 7A is a graph showing a first example of power amplifier supplyvoltage versus time.

FIG. 7B is a graph showing a second example of power amplifier supplyvoltage versus time.

FIG. 8 is a schematic diagram of another embodiment of a communicationsystem.

FIG. 9A is a schematic diagram of one embodiment of a packaged module.

FIG. 9B is a schematic diagram of a cross-section of the packaged moduleof FIG. 9A taken along the lines 9B-9B.

FIG. 10 is a schematic diagram of one embodiment of a phone board.

FIG. 11 is a schematic diagram of one embodiment of a mobile device.

DETAILED DESCRIPTION OF EMBODIMENTS

The following detailed description of certain embodiments presentsvarious descriptions of specific embodiments. However, the innovationsdescribed herein can be embodied in a multitude of different ways, forexample, as defined and covered by the claims. In this description,reference is made to the drawings where like reference numerals canindicate identical or functionally similar elements. It will beunderstood that elements illustrated in the figures are not necessarilydrawn to scale. Moreover, it will be understood that certain embodimentscan include more elements than illustrated in a drawing and/or a subsetof the elements illustrated in a drawing. Further, some embodiments canincorporate any suitable combination of features from two or moredrawings.

Envelope tracking is a technique that can be used to increase poweradded efficiency (PAE) of a power amplifier by efficiently controlling avoltage level of a power amplifier supply voltage in relation to anenvelope of a radio frequency (RF) signal amplified by the poweramplifier. Thus, when the envelope of the RF signal increases, thevoltage supplied to the power amplifier can be increased. Likewise, whenthe envelope of the RF signal decreases, the voltage supplied to thepower amplifier can be decreased to reduce power consumption.

Fast envelope tracking systems for power amplifiers are provided herein.In certain embodiments, an envelope tracking system for a poweramplifier includes a DC-to-DC converter and a differential erroramplifier configured to operate in combination with one another togenerate a power amplifier supply voltage for the power amplifier basedon an envelope of an RF signal amplified by the power amplifier. Theenvelope tracking system further includes a differential envelopeamplifier configured to amplify a differential envelope signal togenerate a single-ended envelope signal that changes in relation to theenvelope of the radio frequency signal. Additionally, the differentialerror amplifier generates an output current operable to adjust a voltagelevel of the power amplifier supply voltage based on comparing thesingle-ended envelope signal to a reference signal.

The differential envelope amplifier can operate in combination with thedifferential error amplifier to provide wide envelope trackingbandwidth, for instance, 180 MHz or more of modulation bandwidth.

In certain implementations, the differential envelope amplifier includesan amplification circuit configured to amplify the differential envelopesignal, and a common mode feedback circuit operable to compensate theamplification circuit for a common mode error arising from a common modevoltage of the differential envelope signal.

Compensating the amplification circuit for common mode error can enhancethe performance of the envelope tracking system by providing rejectionto a common mode component of the differential envelope signal. Forexample, the differential envelope signal can be received from atransceiver over a relatively long connection that is susceptible tonoise. By including a differential envelope amplifier with common modeerror compensation, the envelope tracking system can operate with robustperformance.

In certain implementations, the differential envelope amplifier includesan amplification circuit including a first differential input forreceiving the differential envelope signal and a second differentialinput configured to receive a differential compensation signal from thecommon mode feedback circuit. Additionally, the common mode feedbackcircuit can provide feedback from an output of the amplification circuitto the second differential input to provide common mode errorcompensation.

To provide further noise rejection, the envelope tracker can furtherinclude a differential input filter for filtering the differentialenvelope signal prior to amplification by the amplification circuit.

In certain implementations, the differential error amplifier includes apair of inputs of different input impedance to aid in providing fasttracking. For example, the differential error amplifier can include afirst input with high input impedance that receives a reference voltageand a second input with low input impedance that receives thesingle-ended envelope signal.

Implementing the differential error amplifier in this manner can enhanceenvelope tracking speed. For example, the second input can source orsink a relatively large current to quickly charge or discharge internalcapacitances of the differential error amplifier to provide a fasttransient response. In contrast, when using an amplifier having a pairof inputs with high input impedance, a resistor-capacitor (RC) timeconstant associated with charging and discharging capacitances can berelatively large. Thus, such envelope trackers can operate with a delaythat degrades the envelope tracking system's bandwidth.

In certain configurations, the second input of the differential erroramplifier that receives the envelope signal can be of much lower inputimpedance the first input that receives the reference voltage. Forexample, the second input can connect to FET drain and/or sourceregions, while the first input can connect to a gate region of muchhigher impedance. In one example, an input impedance of the second inputis at least ten times lower than an input impedance of the first input.

In certain implementations, the differential error amplifier alsogenerates a sense signal, such as a sense current that tracks the outputcurrent. The sense current can be used in a variety of ways to enhancethe performance of the envelope tracking system. For example, theDC-to-DC current can use the sense current in part to generate aregulated voltage, which is combined with the output current of thedifferential error amplifier to generate the power amplifier supplyvoltage.

FIG. 1 is a schematic diagram of one embodiment of a communicationsystem 50 for transmitting RF signals. The communication system 50includes a battery 1, an envelope tracker 2, a power amplifier 3, adirectional coupler 4, a duplexing and switching circuit 5, an antenna6, a baseband processor 7, a signal delay circuit 8, a digitalpre-distortion (DPD) circuit 9, an I/O modulator 10, an observationreceiver 11, an intermodulation detection circuit 12, an envelope delaycircuit 21, a coordinate rotation digital computation (CORDIC) circuit22, a shaping circuit 23, a digital-to-analog converter 24, and areconstruction filter 25.

The communication system 50 of FIG. 1 illustrates one example of an RFsystem that can include an envelope tracking system implemented inaccordance with one or more features of the present disclosure. However,the teachings herein are applicable to RF systems implemented in a widevariety of ways.

The baseband processor 7 operates to generate an in-phase (I) signal anda quadrature-phase (Q) signal, which correspond to signal components ofa sinusoidal wave or signal of a desired amplitude, frequency, andphase. For example, the I signal and the Q signal provide an equivalentrepresentation of the sinusoidal wave. In certain implementations, the Iand Q signals are outputted in a digital format. The baseband processor7 can be any suitable processor for processing baseband signals. Forinstance, the baseband processor 7 can include a digital signalprocessor, a microprocessor, a programmable core, or any combinationthereof.

The signal delay circuit 8 provides adjustable delay to the I and Qsignals to aid in controlling relative alignment between thedifferential envelope signal ENV_p, ENV_n provided to the envelopetracker 2 and the RF signal RF_(IN) provided to the power amplifier 3.The amount of delay provided by the signal delay circuit 8 is controlledbased on amount of intermodulation in adjacent bands detected by theintermodulation detection circuit 12.

The DPD circuit 9 operates to provide digital shaping to the delayed Iand Q signals from the signal delay circuit 8 to generate digitallypre-distorted I and Q signals. In the illustrated embodiment, the DPDprovided by the DPD circuit 9 is controlled based on amount ofintermodulation detected by the intermodulation detection circuit 12.The DPD circuit 9 serves to reduce a distortion of the power amplifier 3and/or to increase the efficiency of the power amplifier 3.

The I/O modulator 10 receives the digitally pre-distorted I and Qsignals, which are processed to generate the RF signal RF_(IN). Forexample, the I/O modulator 10 can include DACs configured to convert thedigitally pre-distorted I and Q signals into an analog format, mixersfor upconverting the analog I and Q signals to radio frequency, and asignal combiner for combining the upconverted I and Q signals into theRF signal RF_(IN). In certain implementations, the I/O modulator 10 caninclude one or more filters configured to filter frequency content ofsignals processed therein.

The envelope delay circuit 21 delays the I and Q signals from thebaseband processor 7. Additionally, the CORDIC circuit 22 processes thedelayed I and Q signals to generate a digital envelope signalrepresenting an envelope of the RF signal RF_(IN). Although FIG. 1illustrates an implementation using the CORDIC circuit 22, an envelopesignal can be obtained in other ways.

The shaping circuit 23 operates to shape the digital envelope signal toenhance the performance of the communication system 50. In certainimplementations, the shaping circuit 23 includes a shaping table thatmaps each level of the digital envelope signal to a corresponding shapedenvelope signal level. Envelope shaping can aid in controllinglinearity, distortion, and/or efficiency of the power amplifier 3.

In the illustrated embodiment, the shaped envelope signal is a digitalsignal that is converted by the DAC 24 to a differential analog envelopesignal. Additionally, the differential analog envelope signal isfiltered by the reconstruction filter 25 to generate a differentialenvelope signal ENV_p, ENV_n suitable for use by a differential envelopeamplifier of the envelope tracker 2. In certain implementations, thereconstruction filter 25 includes a differential low pass filter.

With continuing reference to FIG. 1 , the envelope tracker 2 receivesthe envelope signal from the reconstruction filter 25 and a batteryvoltage V_(BATT) from the battery 1, and uses the differential envelopesignal ENV_p, ENV_n to generate a power amplifier supply voltageV_(CC_PA) for the power amplifier 3 that changes in relation to theenvelope of the RF signal RF_(IN). The power amplifier 3 receives the RFsignal RF_(IN) from the I/O modulator 10, and provides an amplified RFsignal RF_(OUT) to the antenna 6 through the duplexing and switchingcircuit 5, in this example.

The directional coupler 4 is positioned between the output of the poweramplifier 3 and the input of the duplexing and switching circuit 5,thereby allowing a measurement of output power of the power amplifier 3that does not include insertion loss of the duplexing and switchingcircuit 5. The sensed output signal from the directional coupler 4 isprovided to the observation receiver 11, which can include mixers forproviding down conversion to generate downconverted I and Q signals, andDACs for generating I and Q observation signals from the downconverted Iand Q signals.

The intermodulation detection circuit 12 determines an intermodulationproduct between the I and Q observation signals and the I and Q signalsfrom the baseband processor 7. Additionally, the intermodulationdetection circuit 12 controls the DPD provided by the DPD circuit 9and/or a delay of the signal delay circuit 8 to control relativealignment between the differential envelope signal ENV_p, ENV_n and theRF signal RF_(IN). In another embodiment, the intermodulation detectioncircuit 12 additionally or alternatively controls a delay of the signaldelay circuit 21.

By including a feedback path from the output of the power amplifier 3and baseband, the I and Q signals can be dynamically adjusted tooptimize the operation of the communication system 50. For example,configuring the communication system 50 in this manner can aid inproviding power control, compensating for transmitter impairments,and/or in performing DPD.

Although illustrated as a single stage, the power amplifier 3 caninclude one or more stages. Furthermore, the teachings herein areapplicable to communication systems including multiple power amplifiers.

FIGS. 2A, 2B, and 3 depict schematic diagram of various embodiments ofenvelope tracking systems for a power amplifier. The envelope trackingsystems of these embodiments provide fast envelope tracking. However,the teachings herein are applicable to envelope trackers implemented ina wide variety of ways. Accordingly, other implementations are possible.

FIG. 2A is a schematic diagram of one embodiment of an envelope trackingsystem 60 for a power amplifier 51. The envelope tracking system 60includes a DC-to-DC converter 52, a differential error amplifier 53, anAC combiner 54, a feedback circuit 55, and a differential envelopeamplifier 59.

The power amplifier 51 amplifies an RF input signal RF_(IN) to generatean RF output signal RF_(OUT). The envelope tracking system 60 receivesthe differential envelope signal ENV_p, ENV_n, which changes in relationto an envelope of the RF input signal RF_(IN).

The differential envelope amplifier 59 amplifies the differentialenvelope signal ENV_p, ENV_n to generate a single-ended envelope signalENV that changes in relation to the envelope.

In the illustrated embodiment, the DC-to-DC converter 52 operates togenerate a regulated voltage VREG based on a DC-to-DC reference voltageV_(REF′) and a sense signal SENSE from the differential error amplifier53. The DC-to-DC converter 52 can be implemented in a wide variety ofways including, but not limited to, using a buck converter, a boostconverter, or a buck-boost converter. The DC-to-DC converter 52 is alsoreferred to herein as a switching regulator.

The sense signal SENSE serves to track changes in the single-endedenvelope signal ENV. For example, the sense signal SENSE can change inrelation to an output current of the differential error amplifier 53.However, other implementations are possible.

The differential error amplifier 53 includes a first input that receivesa reference voltage V_(REF) and a second input that receives thesingle-ended envelope signal ENV from the differential envelopeamplifier 59. In the illustrated embodiment, the first input is anon-inverting input and the second input is an inverting input. However,other implementations are possible. The differential error amplifier 53is also referred to herein as a high bandwidth amplifier.

The differential error amplifier 53 further includes an output that iselectrically connected to the second input via the feedback circuit 55.The feedback circuit 55 can be implemented in a wide variety of ways. Inone example, the feedback circuit 55 includes at least one of a resistoror a capacitor, for instance, a parallel combination of a resistor and acapacitor.

The AC combiner 54 operates to combine the output of the DC-to-DCconverter 52 and the output of the differential error amplifier 53 togenerate a power amplifier supply voltage V_(CC_PA) for the poweramplifier 51.

In certain implementations, the inverting input of the differentialerror amplifier 53 that receives the single-ended envelope signal ENV isimplemented with lower input impedance relative to the non-invertinginput that receives the reference voltage V_(REF). For example, withrespect to internal amplification circuitry of the differential erroramplifier 53, the inverting input can connect to FET drain and/or sourceregions, while the non-inverting input can connect to a gate region ofmuch higher impedance.

By providing the single-ended envelope signal ENV to a low impedanceinput, a relatively large current can be sourced or sunk as needed toquickly charge or discharge internal capacitances of the differentialerror amplifier 53. In contrast, when using a pair of inputs with highinput impedance, an RC time constant associated with charging anddischarging capacitances can be relatively large.

The illustrated envelope tracking system 60 also includes thedifferential envelope amplifier 59, which provides a number ofadvantages. For example, including the differential envelope amplifier59 provides superior rejection of noise and/or higher gain for rapidenvelope tracking. In certain implementations, the differential envelopeamplifier 59 is further implemented with a common mode feedback circuitfor reducing a common mode error arising from a common mode voltage ofthe differential envelope signal ENV_p, ENV_n.

Accordingly, the combination of the differential envelope amplifier 59and differential error amplifier 53 can provide fast envelope tracking,for instance, 180 MHz or more of modulation bandwidth.

FIG. 2B is a schematic diagram of another embodiment of an envelopetracking system 70 for a power amplifier 51. The envelope trackingsystem 70 includes a DC-to-DC converter 52, a differential erroramplifier 53, an AC combiner 54, a feedback circuit 55, a current source56, a DC tracking circuit 57, and a differential envelope amplifier 59.

The envelope tracking system 70 of FIG. 2B is similar to the envelopetracking system 60 of FIG. 2A, except that the envelope tracking system70 further includes the current source 56 and the DC tracking circuit57.

The current source 56 is electrically connected to the second input ofthe differential error amplifier 53, and provides a current that iscontrolled by the DC tracking circuit 57. The DC tracking circuit 57monitors the AC combiner 54, such as one or more internal currentsand/or voltages, and adjusts the current of the current source 56 tomaintain suitable DC biasing levels.

FIG. 3 is a schematic diagram of another embodiment of an envelopetracking system 160 for a power amplifier 51. The envelope trackingsystem 160 includes a differential envelope amplifier 101, a DC-to-DCconverter 102, a differential error amplifier 103, an AC combiner 104, acurrent source 106, a DC tracking circuit 107, a DAC 109, a firstfeedback resistor 111, a second feedback resistor 112, and an inputresistor 113.

The DC-to-DC converter 102 includes a switcher 121, an inductor 122, anoutput capacitor 123, a hysteretic current comparator 124, and a controlvoltage adder 125. The DAC 109 receives a digital reference signalREF_DAC, which controls a voltage of the DC-to-DC reference voltageV_(REF′) generated by the DAC 109. In certain implementations, thedigital reference signal REF_DAC is received over an interface, forinstance, a serial bus such as the Mobile Industry Peripheral Interface(MIPI) Radio Frequency Front-End Control Interface (RFFE) bus 651 ofFIG. 8 .

The hysteretic current comparator 124 processes a sense currentI_(SENSE) from the differential error amplifier 103 to generate acorrection voltage V_(COR). The control voltage adder 125 adds thecorrection voltage V_(COR), a DC tracking correction voltage from the DCtracking circuit 107, and the DC-to-DC reference voltage V_(REF′) togenerate a control voltage V_(CTL) of the switcher 121. The switcher 121receives a battery voltage V_(BATT) and a ground voltage, and controls acurrent flowing through the inductor 122 over time to control a voltagelevel of a regulated voltage V_(REG) at the output of the DC-to-DCconverter 102.

Including the hysteretic current comparator 124 aids in controlling thevoltage level of the regulated voltage V_(REG) based on the sensecurrent I_(SENSE) so as to reduce an average output current of thedifferential envelope amplifier 103. Since the DC-to-DC converter 102can have a higher efficiency than the differential error amplifier 103,reducing the average output current of the differential error amplifier103 can improve the overall efficiency of the envelope tracking system160.

The AC combiner 104 includes an inductor 131 and an AC couplingcapacitor 132. As shown in FIG. 3 , the inductor 131 is connectedbetween the regulated voltage V_(REG) and the power amplifier supplyvoltage V_(CC_PA), and the AC coupling capacitor 132 is connectedbetween the power amplifier supply voltage V_(CC_PA) and the output ofthe differential error amplifier 103.

In the illustrated embodiment, the first feedback resistor 111 isconnected between the output and inverting input of the differentialerror amplifier 103. Additionally, the second feedback resistor 112 isconnected between the power amplifier supply voltage V_(CC_PA) and theinverting input of the differential error amplifier 103. Although oneexample of feedback for a differential envelope amplifier is shown, awide variety of implementations of feedback can be used.

With continuing reference to FIG. 3 , the envelope signal ENV isprovided to the inverting input of the differential error amplifier 103via the input resistor 113. Additionally, the non-inverting input of thedifferential error amplifier 103 receives the reference voltage V_(REF).The differential error amplifier 103 generates an output current, whichis provided to the power amplifier supply voltage V_(CC_PA) via thecapacitor 132 to provide voltage level adjustment to the power amplifiersupply voltage V_(CC_PA). In the illustrated embodiment, thedifferential error amplifier 103 also generates the sense currentI_(SENSE), which changes in relation to the amplifier's output current.

In certain implementations, the inverting input of the differentialerror amplifier 103 that receives the single-ended envelope signal ENVis implemented with lower input impedance relative to the non-invertinginput that receives the reference voltage V_(REF). Thus, thedifferential error amplifier 103 provides fast envelope tracking.

The current source 106 is electrically connected between the invertinginput of the differential error amplifier 103 and ground. The DCtracking circuit 107 controls a current of the current source 106 basedon a voltage across the capacitor 132. In particular, the DC trackingcircuit 107 controls the current to maintain the voltage across thecapacitor 132 relatively constant, thereby helping to maintainsufficient voltage headroom and suitable DC biasing at the output of thedifferential error amplifier 103.

In certain implementations, the DC tracking circuit 107 serves tocontrol the current of the current source 106 such that the voltageacross the capacitor 132 is about equal to a desired voltage, such as areference voltage. Thus, the DC tracking circuit 107 adjusts the currentof the current source 106 to provide DC tracking. As shown in FIG. 3 ,the DC tracking circuit 107 also provides a DC tracking correctionvoltage to the DC-to-DC converter 102, in certain implementations.

FIG. 4 is a schematic diagram of one embodiment of a differentialenvelope amplifier 210 for an envelope tracking system. The differentialenvelope amplifier 210 includes an amplification circuit 201, a commonmode feedback circuit 202, and a differential input filter 203.

The differential envelope amplifier 210 of FIG. 4 illustrates oneembodiment of the differential envelope amplifier of FIGS. 2A-3 .Although one example of a differential envelope amplifier is shown, thedifferential envelope amplifier of FIGS. 2A-3 can be implemented inother ways.

The differential input filter 203 receives a differential envelopesignal ENV_p, ENV_n, and filters the differential envelope signal togenerate a filtered differential envelope signal.

The amplification circuit 201 includes a first differential input thatreceives the filtered differential envelope signal from the differentialinput filter 203 and a second differential input that receives adifferential compensation signal from the common mode feedback circuit202. The amplification circuit 201 includes an output that generates asingle-ended envelope signal ENV.

As shown in FIG. 4 , the common mode feedback circuit 202 is connectedbetween the output of the amplification circuit 201 and the seconddifferential input of the amplification circuit 201. The common modefeedback circuit 202 provides single-ended to differential signalconversion, in this example.

The common mode feedback circuit 202 provides feedback that compensatesthe amplification circuit 201 for an error arising from a common modevoltage of the differential envelope signal ENV_p, ENV_n.

FIG. 5 is a schematic diagram of one embodiment of a differentialenvelope amplifier 240 for an envelope tracking system. The differentialenvelope amplifier 240 includes an amplification circuit 211, a commonmode feedback circuit 212, and a differential input filter 213.

The differential envelope amplifier 240 of FIG. 5 is similar to thedifferential envelope amplifier 210 of FIG. 4 , except that thedifferential envelope amplifier 240 includes specific implementations ofcircuitry. Although one example of circuitry is shown, a differentialenvelope amplifier can be implemented in other ways.

In the illustrated embodiment, the amplification circuit 211 includes afirst differential input, a second differential input, and an output.The first differential input is a voltage input associated with a firsttransconductance Gm_IN, and the second differential input is a voltageinput associated with a second transconductance Gm_FBK. In certainimplementations, Gm_IN is greater than Gm_FBK.

With continuing reference to FIG. 5 , the common mode feedback circuit212 includes a first resistor 221 and a second resistor 222, whichoperate as a voltage divider that generates a divided voltage V_(DIV).The first resistor 221 and the second resistor 222 are connected inseries between the output of the amplification circuit 211 and areference voltage, such as ground. The common mode feedback circuit 212includes a capacitor 224 in parallel with the first resistor 221. Thecommon mode feedback circuit 212 further includes a third resistor 223and a current source 225 connected in series between a supply voltageand ground. The second differential input of the amplification circuit221 compares a voltage V_(R) across the third resistor 223 to thedivided voltage V_(DIV) generated by the first and second resistor 221,222. In certain implementations the current source 225 is controllable(for instance, variable and/or programmable) to control a common modesetting of the common mode feedback circuit 212.

The common mode feedback circuit 212 operates to provide feedback thatcontrols an output DC bias point or level of the amplification circuit211, thereby reducing or eliminating an impact of a common mode voltageof the differential envelope signal ENV_P, ENV_n.

In the illustrated embodiment, the differential input filter 213includes a first filter resistor 231, a second filter resistor 232, anda filter capacitor 233. The differential input filter 213 provides lowpass filtering to the differential envelope signal ENV_p, ENV_n, andprovides the filtered differential envelope signal to the firstdifferential input of the amplification circuit 211.

FIG. 6 is a schematic diagram of one embodiment of an amplificationcircuit 400 for the differential envelope amplifiers of FIGS. 4 and 5 .Although one example of a suitable amplification circuit is shown, adifferential envelope amplifier can include amplification circuitryimplemented in a wide variety of ways.

As shown in FIG. 6 , the differential amplification circuit 400 includesa first pair of p-type field effect transistors (PFETs) 301-302 foramplifying a first differential input IN_(p), IN_(n). The first pair ofPFETs 301-302 is biased by a first pair of current sources 321-322 (eachproviding a current I_(BIAS), in this example), and includes a firstresistor 331 of resistance R for coupling the source of the PFET 301 tothe source of the PFET 302. The differential amplification circuit 400further includes a second pair of PFETs 303-304 for amplifying a seconddifferential input V_(INp_fd), V_(INn_fd), corresponding to adifferential common mode compensation signal. The second pair of PFETs303-304 is biased by a second pair of current sources 323-324 (alsoproviding a current I_(BIAS), in this example), and includes a secondresistor 332 (also of resistance R, in this example) for coupling thesource of the PFET 303 to the source of the PFET 304.

Currents from the first pair of PFETs 301-302 and the second pair ofPFETs 303-304 are combined using folded cascode circuitry that includescurrent sources 325-326, n-type field effect transistors (NFETs)311-312, and PFETs 313-314. In this example, the gates of NFETs 311-312are controlled by a bias voltage V_(BIAS).

The amplification circuit 400 further includes a push-pull output stageincluding NFET 317, PFET 318, a current source 327, and a class AB biascircuit 328. As shown in FIG. 6 , the current source 327 provides acurrent I_(BIAS_AB) to the class AB bias circuit 328, which biases theNFET 317 and PFET 318 to provide enhanced bandwidth.

FIG. 7A is a graph 617 showing a first example of power amplifier supplyvoltage versus time. The graph 617 illustrates the voltage of an RFsignal 611, the RF signal's envelope 612, and a power amplifier supplyvoltage 613 versus time. The graph 617 corresponds to one example ofwaveforms for an implementation in which the power amplifier supplyvoltage 613 is substantially fixed.

It can be important that the power amplifier supply voltage 613 of apower amplifier has a voltage greater than that of the RF signal 611.For example, powering a power amplifier using a power amplifier supplyvoltage that has a magnitude less than that of the RF signal can clipthe RF signal, thereby creating signal distortion and/or other problems.Thus, it can be important the power amplifier supply voltage 613 begreater than that of the envelope 612. However, it can be desirable toreduce a difference in voltage between the power amplifier supplyvoltage 613 and the envelope 612 of the RF signal 611, as the areabetween the power amplifier supply voltage 613 and the envelope 612 canrepresent lost energy, which can reduce battery life and increase heatgenerated in a wireless device.

FIG. 7B is a graph 618 showing a second example of power amplifiersupply voltage versus time. The graph 618 illustrates the voltage of anRF signal 611, the RF signal's envelope 612, and a power amplifiersupply voltage 614 versus time. The graph 618 corresponds to one exampleof waveforms for an implementation in which the power amplifier supplyvoltage 614 is generated by envelope tracking.

In contrast to the power amplifier supply voltage 613 of FIG. 7A, thepower amplifier supply voltage 614 of FIG. 7B changes in relation to theenvelope 612 of the RF signal 611. The area between the power amplifiersupply voltage 614 and the envelope 612 in FIG. 7B is less than the areabetween the power amplifier supply voltage 613 and the envelope 612 inFIG. 7A, and thus the graph 618 of FIG. 7B can be associated with anenvelope tracking system having greater energy efficiency.

FIG. 8 is a schematic diagram of another embodiment of a communicationsystem 660. The communication system 660 includes a transceiver 641, apower amplifier module 642, a transmit filter module 643, a receivefilter module 644, a low noise amplifier (LNA) module 645, an antennaswitch module 646, a coupler module 647, a sensor module 648, a powermanagement module 649, an antenna 650, and a MIPI RFFE bus 651.

As shown in FIG. 8 , various components of the communication system 660are interconnected by the MIPI RFFE bus 651. Additionally, thetransceiver 641 includes a master device of the MIPI RFFE bus 651, andeach of the RF components includes a slave device of the MIPI RFFE bus651. The master device of the transceiver 641 sends control commandsover the MIPI RFFE bus 651 to configure the communication system 660during initialization and/or while operational.

The power amplifier module 642 can include one or more power amplifiers.As shown in FIG. 8 , the power amplifier module 642 receives one or morepower amplifier supply voltages from the power management module 649.The power management module 649 can include an envelope tracker thatgenerates at least one power amplifier supply voltage, and that isimplemented in accordance with the teachings herein.

Although FIG. 8 illustrates one example of a communication systemincluding a power management module and a power amplifier module, theteachings herein are applicable to communication systems implemented ina wide variety of ways.

FIG. 9A is a schematic diagram of one embodiment of a packaged module700. FIG. 9B is a schematic diagram of a cross-section of the packagedmodule 700 of FIG. 9A taken along the lines 9B-9B. The packaged module700 illustrates an example of a module that can include circuitryimplemented in accordance with one or more features of the presentdisclosure.

The packaged module 700 includes a first die 701, a second die 702,surface mount components 703, wirebonds 708, a package substrate 720,and encapsulation structure 740. The package substrate 720 includes pads706 formed from conductors disposed therein. Additionally, the dies 701,702 include pads 704, and the wirebonds 708 have been used to connectthe pads 704 of the dies 701, 702 to the pads 706 of the packagesubstrate 720.

In certain implementations, the dies 701, 702 are manufactured usingdifferent processing technologies. In one example, the first die 701 ismanufactured using a compound semiconductor process, and the second die702 is manufactured using a silicon process. Although an example withtwo dies is shown, a packaged module can include more or fewer dies.

The packaging substrate 720 can be configured to receive a plurality ofcomponents such as the dies 701, 702 and the surface mount components703, which can include, for example, surface mount capacitors and/orinductors.

As shown in FIG. 9B, the packaged module 700 is shown to include aplurality of contact pads 732 disposed on the side of the packagedmodule 700 opposite the side used to mount the dies 701, 702.Configuring the packaged module 700 in this manner can aid in connectingthe packaged module 700 to a circuit board such as a phone board of awireless device. The example contact pads 732 can be configured toprovide RF signals, bias signals, ground, and/or supply voltage(s) tothe dies 701, 702 and/or the surface mount components 703. As shown inFIG. 9B, the electrically connections between the contact pads 732 andthe dies 701, 702 can be facilitated by connections 733 through thepackage substrate 720. The connections 733 can represent electricalpaths formed through the package substrate 720, such as connectionsassociated with vias and conductors of a multilayer laminated packagesubstrate.

In some embodiments, the packaged module 700 can also include one ormore packaging structures to, for example, provide protection and/orfacilitate handling of the packaged module 700. Such a packagingstructure can include overmold or encapsulation structure 740 formedover the packaging substrate 720 and the components and die(s) disposedthereon.

It will be understood that although the packaged module 700 is describedin the context of electrical connections based on wirebonds, one or morefeatures of the present disclosure can also be implemented in otherpackaging configurations, including, for example, flip-chipconfigurations.

FIG. 10 is a schematic diagram of one embodiment of a phone board 750.The phone board 750 includes an envelope tracking module 752 and a poweramplifier module 751 attached thereto. In certain configurations, thepower amplifier module 751 and/or the envelope tracking module 752 areimplemented using a module similar to that of the module 700 shown inFIGS. 9A-9B. As shown in FIG. 10 , the envelope tracking module 752provides a power amplifier supply voltage V_(CC_PA) to the poweramplifier module 751. Additionally, the envelope tracking module 752controls the power amplifier supply voltage V_(CC_PA) to change inrelation to the envelope of an RF signal amplified by the poweramplifier module 751.

Although not illustrated in FIG. 10 for clarity, the phone board 750typically includes additional components and structures.

FIG. 11 is a schematic diagram of one embodiment of a mobile device 800.The mobile device 800 includes a baseband system 801, a transceiver 802,a front end system 803, antennas 804, a power management system 805, amemory 806, a user interface 807, and a battery 808.

The mobile device 800 can be used communicate using a wide variety ofcommunications technologies, including, but not limited to, 2G, 3G, 4G(including LTE, LTE-Advanced, and LTE-Advanced Pro), 5G, WLAN (forinstance, Wi-Fi), WPAN (for instance, Bluetooth and ZigBee), WMAN (forinstance, WiMax), and/or GPS technologies.

The transceiver 802 generates RF signals for transmission and processesincoming RF signals received from the antennas 804. It will beunderstood that various functionalities associated with the transmissionand receiving of RF signals can be achieved by one or more componentsthat are collectively represented in FIG. 11 as the transceiver 802. Inone example, separate components (for instance, separate circuits ordies) can be provided for handling certain types of RF signals.

The front end system 803 aids is conditioning signals transmitted toand/or received from the antennas 804. In the illustrated embodiment,the front end system 803 includes power amplifiers (PAs) 811, low noiseamplifiers (LNAs) 812, filters 813, switches 814, and duplexers 815.However, other implementations are possible.

For example, the front end system 803 can provide a number offunctionalizes, including, but not limited to, amplifying signals fortransmission, amplifying received signals, filtering signals, switchingbetween different bands, switching between different power modes,switching between transmission and receiving modes, duplexing ofsignals, multiplexing of signals (for instance, diplexing ortriplexing), or some combination thereof.

In certain implementations, the mobile device 800 supports carrieraggregation, thereby providing flexibility to increase peak data rates.Carrier aggregation can be used for both Frequency Division Duplexing(FDD) and Time Division Duplexing (TDD), and may be used to aggregate aplurality of carriers or channels. Carrier aggregation includescontiguous aggregation, in which contiguous carriers within the sameoperating frequency band are aggregated. Carrier aggregation can also benon-contiguous, and can include carriers separated in frequency within acommon band or in different bands.

The antennas 804 can include antennas used for a wide variety of typesof communications. For example, the antennas 804 can include antennasassociated transmitting and/or receiving signals associated with a widevariety of frequencies and communications standards.

In certain implementations, the antennas 804 support MIMO communicationsand/or switched diversity communications. For example, MIMOcommunications use multiple antennas for communicating multiple datastreams over a single radio frequency channel. MIMO communicationsbenefit from higher signal to noise ratio, improved coding, and/orreduced signal interference due to spatial multiplexing differences ofthe radio environment. Switched diversity refers to communications inwhich a particular antenna is selected for operation at a particulartime. For example, a switch can be used to select a particular antennafrom a group of antennas based on a variety of factors, such as anobserved bit error rate and/or a signal strength indicator.

The mobile device 800 can operate with beamforming in certainimplementations. For example, the front end system 803 can include phaseshifters having variable phase controlled by the transceiver 802.Additionally, the phase shifters are controlled to provide beamformation and directivity for transmission and/or reception of signalsusing the antennas 804. For example, in the context of signaltransmission, the phases of the transmit signals provided to theantennas 804 are controlled such that radiated signals from the antennas804 combine using constructive and destructive interference to generatean aggregate transmit signal exhibiting beam-like qualities with moresignal strength propagating in a given direction. In the context ofsignal reception, the phases are controlled such that more signal energyis received when the signal is arriving to the antennas 804 from aparticular direction. In certain implementations, the antennas 804include one or more arrays of antenna elements to enhance beamforming.

The baseband system 801 is coupled to the user interface 807 tofacilitate processing of various user input and output (I/O), such asvoice and data. The baseband system 801 provides the transceiver 802with digital representations of transmit signals, which the transceiver802 processes to generate RF signals for transmission. The basebandsystem 801 also processes digital representations of received signalsprovided by the transceiver 802. As shown in FIG. 11 , the basebandsystem 801 is coupled to the memory 806 of facilitate operation of themobile device 800.

The memory 806 can be used for a wide variety of purposes, such asstoring data and/or instructions to facilitate the operation of themobile device 800 and/or to provide storage of user information.

The power management system 805 provides a number of power managementfunctions of the mobile device 800. The power management system 805 caninclude an envelope tracker 860 implemented in accordance with one ormore features of the present disclosure.

As shown in FIG. 11 , the power management system 805 receives a batteryvoltage form the battery 808. The battery 808 can be any suitablebattery for use in the mobile device 800, including, for example, alithium-ion battery.

Conclusion

Some of the embodiments described above have provided examples inconnection with mobile devices. However, the principles and advantagesof the embodiments can be used for any other systems or apparatus thathave needs for envelope tracking.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Likewise, the word “connected”, as generally used herein, refers to twoor more elements that may be either directly connected, or connected byway of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the above Detailed Description using the singular or pluralnumber may also include the plural or singular number respectively. Theword “or” in reference to a list of two or more items, that word coversall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A mobile device comprising: a transceiverconfigured to generate a radio frequency transmit signal; a front endcircuit including a power amplifier configured to amplify the radiofrequency transmit signal; and a power management circuit including anenvelope tracker configured to generate a power amplifier supply voltageof the power amplifier, the envelope tracker including a differentialenvelope amplifier having an input filter with at least a first resistorconnected between a non-inverting input and a non-inverting output, asecond resistor connected between an inverting input and an invertingoutput, and a capacitor connected between the non-inverting output andthe inverting output to generate an envelope signal that changes inrelation to an envelope of the radio frequency transmit signal.
 2. Themobile device of claim 1 wherein the input filter is a low pass filter.3. The mobile device of claim 2 further comprising a differential erroramplifier configured to generate an output current based on comparingthe envelope signal to a reference signal, the output current operableto adjust a voltage level of the power amplifier supply voltage.
 4. Themobile device of claim 3 wherein the differential error amplifier isconfigured to provide a sense current to a switching regulator, thesense current configured to change in relation to the output current. 5.The mobile device of claim 4 wherein the switching regulator isconfigured to generate a regulated voltage.
 6. The mobile device ofclaim 3 wherein the differential error amplifier includes a first inputconfigured to receive a reference voltage and a second input configuredto receive the envelope signal, the second input having lower inputimpedance than the first input to thereby widen envelope trackingbandwidth.
 7. The mobile device of claim 6 further comprising a combinerconfigured to combine an output current of the differential erroramplifier and a regulated voltage to generate the power amplifier supplyvoltage.
 8. The mobile device of claim 1 wherein the envelope trackerfurther includes a first differential input configured to receive theenvelope signal, and a second differential input, the envelope trackerfurther including a common-mode feedback circuit connected between theoutput and the second differential input.
 9. The mobile device of claim8 wherein the common-mode feedback circuit includes a voltage dividerconnected between the output of the differential envelope amplifier andground and configured to generate a divided voltage, the seconddifferential input receiving a voltage difference between a controllablevoltage and the divided voltage.
 10. The mobile device of claim 9wherein the common-mode feedback circuit further includes a controllablecurrent source and a resistor in series and configured to generate thecontrollable voltage.
 11. An envelope tracking system for a mobiledevice, the envelope tracking system comprising: a power amplifierconfigured to amplify a radio frequency signal, the power amplifierpowered by a power amplifier supply voltage; and an envelope trackerconfigured to generate the power amplifier supply voltage based on anenvelope of the radio frequency signal indicated by a differentialenvelope signal, the envelope tracker including a differential envelopeamplifier having an input filter with a first resistor connected betweena non-inverting input and a non-inverting output, a second resistorconnected between an inverting input and an inverting output, and acapacitor connected between the non-inverting output and the invertingoutput to generate an envelope signal.
 12. The envelope tracking systemof claim 11 wherein the input filter is a low pass filter.
 13. Theenvelope tracking system of claim 11 further comprising a differentialerror amplifier configured to generate an output current based oncomparing the envelope signal to a reference signal, the output currentoperable to adjust a voltage level of the power amplifier supplyvoltage.
 14. The envelope tracking system of claim 13 wherein thedifferential error amplifier is configured to provide a sense current toa switching regulator, the sense current configured to change inrelation to the output current.
 15. The envelope tracking system ofclaim 14 wherein the switching regulator is configured to generate aregulated voltage.
 16. The envelope tracking system of claim 13 whereinthe differential error amplifier includes a first input configured toreceive a reference voltage and a second input configured to receive theenvelope signal, the second input having lower input impedance than thefirst input to thereby widen envelope tracking bandwidth.
 17. Theenvelope tracking system of claim 16 further comprising a combinerconfigured to combine an output current of the differential erroramplifier and the regulated voltage to generate the power amplifiersupply voltage.
 18. The envelope tracking system of claim 11 wherein theenvelope tracker further includes a first differential input configuredto receive the differential envelope signal, an output that provides theenvelope signal, and a second differential input, the envelope trackerfurther including a common-mode feedback circuit connected between theoutput and the second differential input.
 19. The envelope trackingsystem of claim 18 wherein the common-mode feedback circuit includes avoltage divider connected between the output of the differentialenvelope amplifier and ground and configured to generate a dividedvoltage, the second differential input receiving a voltage differencebetween a controllable voltage and the divided voltage.
 20. The envelopetracking system of claim 19 wherein the common-mode feedback circuitfurther includes a controllable current source and a resistor in seriesand configured to generate the controllable voltage.